In Direct Digital Synthesis (DDS), desired signal behaviour is achieved by converting digital phase data or other variable data to digital amplitude data, for instance by means of look-up tables. The digital amplitude data can then be converted to an analog amplitude by a digital-to-analog converter, or the amplitude data can be further used e.g. for signal phase or frequency processing.
A sine form signal converted from phase data represents a typical DDS conversion. The simplest way to perform the conversion is to store amplitude values of a sine function in a ROM memory (Read Only Memory) functioning as a look-up table, said values being indicated by the phase data. The use of look-up table alone will, however, lead to extensive deployment of memory which, in turn slows down the converter and the frequency of the signal obtained from the converter.
To reduce the need for memory, it is possible to approximate the function according to which the amplitude is to behave. One alternative is to use the Taylor series expansion disclosed in U.S. Pat. No. 4,905,177. From the series expansion are taken into account three first terms, which are used for making the conversion. An impairment of the method is that it requires a third, non-linear term and a multiplier for performing multiplication. Due to such shortcomings, the solution is slow.